Digital-to-analog converters (hereinafter referred to as “DACs”) are widely used for various applications including wireless communication. For example, a wireless communication device typically includes a transmit DAC (“TxDAC”) to convert one or more digital data streams into one or more analog signals. The analog signals are further processed to generate a radio frequency (RF) output signal that is suitable for transmission over a wireless channel. The TxDAC is typically designed to meet stringent dynamic specifications for spurious free dynamic range (SFDR), signal-to-noise ratio (SNR), total harmonic distortion (THD), and the like. These dynamic specifications determine the quality (e.g., spectral purity) of the analog signals from the TxDAC and are normally set such that the RF output signal can meet the overall specifications imposed by the wireless system.
An example of such a DAC is described in U.S. Published Patent Application 20060061499, entitled “High-speed and high-accuracy digital-to-analog converter”, by Seo, published on Mar. 23, 2006, and assigned to the assignee of the present application. The entire contents of U.S. Published Patent Application 20060061499 are herein incorporated by reference.
Referring now to FIG. 1, a block diagram illustrates an embodiment of the high speed DAC 100. The DAC 100 is preferably an N-bit DAC, where N may be any integer value (e.g., N=12). In one embodiment, the N-bit DAC 100 is implemented with two thermometer decoders 101 and 102. Thermometer-decoder 101 controls M most significant bits, and thermometer decoder 102 controls L least significant bits, where L and M may be any integer values such that L+M=N (e.g., L=5, M=7, and N=12). Preferably, the M bit section of DAC 100 includes a latches/drivers 103, switches 105, a current bias 107 and a MSB current source 109, and the L bit section of DAC 100 includes a latches/drivers 104, switches 106, a cascode bias 108 and a LSB current source 110. The DAC 100 includes two differential output signals OUTP (output plus) and OUTM (output minus)
Referring now to FIG. 2, a simplified block diagram illustrates the DAC 100 implemented in a conventional Tx channel of a wireless device. The outputs of the DAC 100 include fully-differential current outputs OUTP and OUTM. This means for a given output power the total DACs' DC current is always constant. This results in constant DC current in succeeding stages of FIG. 2, such as baseband transmit filter 201 and up-conversion mixer 202 because of the inherent property in the current mirroring stage. In the simplified transmit chain of FIG. 2 the transmit filter 201 may be a 1-pole passive baseband (BB) filter, but the order of this filter is generally greater than or equal to 2 (e.g., 2 or 3). “N” represents the current mirror gain of the filter stages. A similar current gain may be needed in the mixer block 202. The constant current outputs of the DAC 100, however, tend to limit the power efficiency in Tx path and tend to limit battery talk time of the wireless device.
Accordingly, there is a need to reduce the DC current of the Tx path in a wireless communication device using a baseband transmit DAC with code dependent DC current.